1. Field of the Invention
The present invention relates generally to the field of fiber optic transceivers and particularly to an optical transceiver having a multipurpose internal serial bus used to accomplish setup, control, monitoring, and safety operations.
2. Description of Related Art
Fiber optic transceivers, otherwise know as optoelectronic transceivers, transmit and receive both light and electronics signals, i.e., they provide for the bi-directional communication of signals between an electrical interface and an optical interface.
FIG. 1 is a schematic representation of the circuitry and components found in various prior-art fiber optic transceivers 100. The fiber optic transceiver 100 includes a circuit board 102 that contains at a minimum a receiver circuit, a transmit circuit, a power connection 104, and a ground connection 106.
The receiver circuit receives relatively small optical signals at an optical detector and amplifies and limits the signals to create a uniform amplitude digital electronic output. The receiver circuit typically consists of a Receiver Optical Subassembly (ROSA) 108, which typically includes a fiber receptacle as well as a photodiode and pre-amplifier (preamp) circuit. The ROSA 108 is in turn connected to a post-amplifier (postamp) integrated circuit (IC) 110, which generates a fixed output swing digital signal which is connected to outside circuitry via RX+ and RX− pins 112. The postamp IC 110 also often provides a digital output signal known as Signal Detect (SD), or Loss of Signal (LOS), indicating the presence or absence of a suitably strong optical input. This Signal Detect (SD) output is provided via a SD output pin 114.
The transmitter circuit, or laser driver circuit, accepts high speed digital data and electrically drives a Light Emitting Diode (LED) or laser diode to create equivalent optical pulses. The transmit circuit typically consists of a Transmitter Optical Subassembly (TOSA) 116 and a laser driver IC 118. The TOSA 116 typically includes a fiber receptacle as well as a laser diode or LED. The laser driver IC 118 typically includes an alternating current (AC) driver to provide AC current to the laser diode or LED. The laser driver IC 118 also typically includes a direct current (DC) driver to provide bias current to the laser diode or LED. The signal inputs for the AC driver are obtained from TX+ and TX− pins 120.
In addition, some optical transceiver standards require additional transceiver functionality. For example, the GigaBit Interface Converter (GBIC) standard requires eye safety and general fault detection functionality. This functionality is used to identify abnormal and potentially unsafe operating parameters and to report these to the user and/or perform laser shutdown, if appropriate. This functionality may be integrated into the laser driver IC 118 itself or into an optional eye safely IC 122. To enable this functionality, TX disable 124 and TX fault 126 pins are provided. The TX disable pin 124 allows the TOSA 116 to be shut off by a host device, while the TX fault pin 124 communicates a fault condition in the laser, or associated laser driver IC 118, to the host device. In addition to this basic description, the GBIC standard includes a series of timing diagrams describing how these controls function and interact with each other to implement reset operations and other actions. The GBIC standard also requires an Electrically Erasable Programmable Read Only Memory (EEPROM) 128 to store standardized serial identification (ID) information that can be read via a serial interface (defined as using the serial interface of the ATMEL AT24C01A family of EEPROM products) consisting of clock 130 and data 132 lines.
In use, various components, such as the laser driver IC 118, typically require individual setup and adjustment. Setup of these components generally occurs each time the optical transceiver is powered-up, as the individual components generally have volatile memories that lose setup data when powered-down. Adjustment, on the other hand, is typically required to account for variations in component characteristics, such as laser diode threshold current and slope efficiency either on a part to part basis or based on temperature. Parameters that are adjusted are: bias current and AC modulation. This setup and adjustment is generally performed on each optical transceiver by adjusting variable resistors or by connecting resistors 134, having factory selected resistance values, to the laser driver IC 118. Additionally, temperature compensation of the bias current and modulation is often required, which is important to key laser characteristics, such as slope efficiency. Temperature compensation can be integrated into the laser driver IC 118 or accomplished through the use of external temperature sensitive elements, such as thermistors 136.
The above described optical transceiver has a number of drawbacks. For example, it is time consuming and costly to detect the variations of the various components, select the correct resistors 134 and thermistors 136 (or adjust the variable resistors), and/or install these on the circuit board 102 of the optical transceiver 100. In addition, such resistors and/or thermistors take up valuable space on the circuit board 102. Moreover, each time additional features are added to the optical transceiver, the complexity of the circuitry increases significantly. For example, in use, as many as twelve connections may be needed to the transmitter circuit, thereby significantly increasing circuitry complexity. Still further, such prior art systems are generally not scalable, i.e., they cannot easily be expanded to allow additional functionality.
More recently, analog signal lines have been introduced to setup and adjust these components. The analog signals lines may include analog inputs 138, such as bias current, modulation amplitude, and disable inputs, or analog outputs 140, such as actual bias, temperature, and output power. However, these analog signals lines do not address many of the drawbacks of the abovementioned prior art. For example, a high number of inputs and outputs are still required, which leads to complex circuitry that takes up a significant amount of space on the circuit board 102. These optical transceivers are still not scalable.
Dedicated digital connections, such as serial input 142 and output 144 connections, to these components have been introduced. An example of a digital output is a fault indicator, e.g., indicating that the transceiver module is running too hot. Optical transceivers employing these dedicated digital connections typically include a controller IC 148 located either within, or outside, the optical transceiver 100. Inputs and/or outputs 150 between the host and the controller IC 148 are typically also provided. However, optical transceivers employing dedicated digital connections do not fully address the abovementioned drawbacks. For example, the circuitry is still highly complex, especially for multiple inputs/outputs. This circuitry, as in other prior art optical transceivers, takes up valuable circuit board space.
In light of the above, an optical transceiver that addresses the abovementioned drawbacks would be highly desirable. In particular, such an optical transceiver should be simple to manufacture and operate; reduce complexity by providing a simple circuitry layout; use a minimal amount of circuit board space; and allow for scalability.